.a #b{
    border-top: solid 10px red;
    padding:10px;
    margin:10px;
    background-color: blue;
    width:200px;
    height:200px;
    position: absolute;
    top:100px;
    left:100px;
} 
.a #c{
    border-top: solid 10px red;
    padding:10px;
    margin:10px;
    background-color: yellow;
    width:400px;
    height:400px;
}